The present invention relates to an automatic purging system and in particular to a system which is adapted for automatically purging heating mantle and poncho assemblies.
In hazardous locations, for example in the processing of certain flammable chemicals such as acetone and toluene, electrical equipment which is not explosion-proof as defined by the National Electrical Code cannot be used. The Code provides, however, that in certain cases, hazards and hazardous locations may be limited or eliminated by adequate positive pressure ventilation from a source of clean air in conjunction with effective safeguards against ventilation failure. Since providing explosion-proof electrical equipment is quite expensive, it is often desirable to resort to the alternative procedure of maintaining positive pressure ventilation.
Standard 496 of the National Fire Protection Association sets forth the procedures for purging enclosures for electrical equipment in hazardous locations. Basically, what the standard requires is that if the positive ventilation or purging should fail as indicated by failure to maintain a static pressure within the enclosure of at least 0.1 inch of water, power shall be cut off to the electrical equipment and the power shall not be turned on again until at least four enclosure volumes of purged gas have passed through the enclosure while maintaining an internal enclosure pressure of at least 0.1 inch of water.
In chemical processing installations, such as the distillation of flammable organic chemicals, the chemicals are normally heated in a flack supported in a heating mantle which includes an electric heating element. The top of the heating mantle and the flask are enclosed by a poncho safety shield. This shield has an opening which fits tightly around the neck of the flask, is sealed against the mantle and is spaced slightly from the flask so as to provide an enclosure capable of pressurization. The metal poncho protects the flask, which is often made of glass, against damage from falling objects and also protects personnel from flying glass and the contained liquid should a flask explode or implode. Furthermore, since the poncho is spaced from the flask, it provides an insulating barrier which substantially reduces heat losses. The poncho also serves as protection for the heating element of the mantle against damage by chemical spillage or washdown of the apparatus. A factor which is important from the standpoint of the present invention is that since the poncho seals tightly against the flask and mantle, the enclosure can be placed under a positive pressure for purging.